1. Field of Invention
The use of redundant high power amplifiers, with more than adequate output power margins, is still commonplace in satellite communication earth terminals.
Redundancy is employed to ensure continuity of service in the event of an amplifier failure, while the power margin provides for an acceptable transmission level during adverse propagation conditions, or loss of transponder gain.
It is often desirable, both for economic and technical reasons, to provide an adequate output power margin by connecting two amplifiers in parallel, each rated at one half the required output power. This is particularly true for satellite earth terminals operating in the higher frequency bands where large amounts of power may not be available by means other than paralleling.
In the case of failure of one amplifier, the available output drops nominally by 3 dB. To the extent that amplifier failure and propagation conditions resulting in fades in excess of 3 dB occur rarely, the probability of a service interruption or severely degraded transmission is very small.
In applications such as transportable stations, it may be important to use the smallest possible antenna. By operating the amplifiers in parallel, the antenna gain requirement can by reduced by 3 dB compared to a conventional design.
In order to implement the conversion from two parallel amplifiers to a single amplifier, the following three conditions must be fulfilled:
(i) power combining of the two amplifiers PA1 (ii) provision for a straight through connection between antenna and either amplifier PA1 (iii) provision to terminate either, or both amplifiers in a dump load
Previously, phase combining with redundancy has been achieved using a combination of couplers, phase shifters, switches and dump loads. However, the cost and size of these combines has been prohibitive due to the number of components required.
A more acceptable solution is the Variable Power Combiner (VPC) which essentially consists of two orthomode tranducers (OMT) joined by a rotating half-wave plate. These combiners also tend to be cumbersome and expensive, and limited to a bandwidth of only about 5% typically.